This invention relates generally to improvements in fluid flow control systems and, more particularly, to a new and improved automatic, highly accurate, positive pressure infusion pump of the syringe pump type, for parenteral administration of medical liquids over a wide range of fluid flow rates.
The usual medical procedure for the gradual parenteral administration of liquids into the human body, such as liquid nutrients, blood or plasma, makes use of apparatus which is commonly referred to in the medical arts as an intravenous administration set. The intravenous set usually comprises a bottle of liquid, normally supported in an inverted position, an intravenous feeding tube, typically of clear plastic, and a suitable valve mechanism, such as a roll clamp, which allows the liquid to drip out of the bottle at a selectively adjustable rate into a transparent drip chamber below the bottle. The drip chamber serves the dual function of allowing a nurse or other attendant to observe the rate at which the liquid drips out of the bottle, and also creates a reservoir for the liquid at the lower end of the drip chamber to insure that no air enters the main feeding tube leading to the patient.
While observation of the rate of drop flow via the drip chamber is a simple way of controlling the amount of liquid fed to a patient over a period of time, its ultimate effectiveness requires that a relatively constant vigil be maintained on the drop flow, lest it cease entirely due to exhaustion of the liquid supply or become a continuous stream and perhaps increase the rate of liquid introduction to the patient to dangerous levels.
By way of example, it has been the general practice in hospitals to have nurses periodically monitor drop flow rate at each intravenous feeding or parenteral infusion station. Such monitoring of drop flow is a tedious and time consuming process, prone to error and associated, possibly serious consequences, and resulting in a substantial reduction of the available time of qualified medical personnel for other important duties. Typically, the nurse monitoring drop flow rate will use a watch to time the number of drops flowing in an interval of one or more minutes, and she will then mentally perform the mathematics necessary to convert the observed data to an appropriate fluid flow rate, e.g., in cubic centimeters per hour or drops per minute. If the calculated flow rate is substantially different than the prescribed rate, the nurse must manully adjust the roll clamp for a new rate, count drops again, and recalculate to measure the new rate.
Obviously, each of the aforedescribed measurements and calculations and flow rate adjustments usually take several minute's time which, when multiplied by the number of stations being monitored and the number of times each station should be monitored per day, can result in a substantial percentage of total personnel time available. In addition, under the pressure of a heavy schedule, the observations and calculations performed by a harried nurse in measuring and adjusting flow rate may not always prove to be reliable and, hence, errors do occur, resulting in undesired, possibly dangerous infusion flow rates.
In addition to the aforedescribed difficulties, the parenteral administration of medical liquids by gravity induced hydrostatic pressure infusion of the liquid from a bottle or other container suspended above a patient, is very susceptible to fluid flow rate variation due to changes in the liquid level in the bottle, changes in temperature, changes in the venous or arterial pressure of the patient, patient movement, and drift in the effective setting of the roll clamp or other valve mechanism pinching the feeding tube. Moreover, there are a number of situations, such as in intensive care, cardiac and peditric patients, or where rather potent drugs are being administered, where the desired fluid flow rate must be capable of precise selection and must not drift beyond certain prescribed limits. In addition, it is extremely important in such situations for medical personnel to be informed of undesirable fluctuations in flow rate, failure of the fluid delivery system for any reason, leakage of the system, or exhaustion of liquid supply when the bottle is emptied.
It will be apparent, therefore, that some of the most critical problems confronting hospital personnel faced with an overwhelming duty schedule and limited time availability are the problems of quickly, easily, reliably and accurately maintaining proper fluid flow rates in the parenteral administration of medical liquids.
In recent years, a number of electrical monitoring systems, drop flow controllers and infusion pumps have been developed to accomplish the various tasks of monitoring and regulating drop flow rates. Some of these devices have also been capable of activating alarms when a potentially dangerous condition exists, thus freeing medical personnel to some extent, for other duties. However, while such monitoring and drop rate control devices have generally served their purpose, they have not always proven entirely satisfactory from the standpoint of cost, complexity, stability, reliability, accuracy, or precision of adjustment over a wide range of selected fluid flow rates. In addition, such systems have sometimes been subject to drift and substantial flow rate variations due to changes in temperature, feeding tube crimps, variations in venous or arterial pressure of the patient, or variations in the height of the bottle or solution level within the bottle. Substantial difficulties have also been experienced particularly in connection with establishing and maintaining accurate flow at very low flow rates.
Positive pressure pumps of the closed loop peristaltic type have been provided which overcome some of the aforementioned difficulties with regard to drift, and accurate flow at low flow rates. However, even such closed loop positive pressure systems only serve to maintain accuracy of flow in terms of stablizing to a preselected drop flow rate, rather than delivering a precise preselected volume of fluid, e.g., in cubic centimeters per hour. The reason for this is that the accuracy of such systems is limited inherently to the accuracy of the size of the drops produced by an intravenous administration set, and the actual drops produced by the latter apparatus can vary from its designated drop size, e.g., by virtue of drip chamber structural variations, by as much as 30 percent.
Positive pressure infusion pumps of the syringe type have also been provided, wherein a syringe having a precise displacement volume is repeatedly filled and emptied on alternate piston strokes during a combined fill stroke and pump stroke operational cycle, so that control of the rate at which the syringe is filled and emptied provides an accurate means for precise fluid volume delivery within a prescribed period of time. However, since a portion of each operating cycle with such syringe type pumps is concerned with filling the syringe, rather than delivering fluid to the patient in a pumping mode, the accuracy of such devices tends to fall off, particularly at very high flow rates, where the fill stroke period is of significantly large duration relative to the pump stroke period. Attempts have been made to provide various types of non-linear calibration for such syringe type systems, in an attempt to compensate for the lost time error due to syringe "fill" time in each complete pumping cycle. However, these efforts, at best, have only reduced the degree of inaccuracy in certain limited flow rate ranges, e.g., at low flow rates, and have failed to provide a uniformly high degree of accuracy over a wide range of flow rates, and particularly at very high fluid flow rates.
Hence, those concerned with the development and use of parenteral fluid administration systems, and particularly those concerned with the design of automatic fluid flow control systems, have long recognized the need for improved, relatively simple, economical, reliable, stable and accurate devices for fluid flow control which obviate the aforementioned difficulties. The present invention provides a new and improved fluid flow control system in the form of a syringe pump which clearly fulfills this need.